Recently, research and development has been actively conducted on a non-aqueous electrolyte battery, such as a lithium-ion secondary battery, as a high energy density battery. The non-aqueous electrolyte battery is expected as a power source for a hybrid vehicle, an electric vehicle, or an uninterruptible power source of a mobile phone base station. However, even when a size of a single battery of a lithium-ion secondary battery is increased, a voltage obtained from the single battery is a low voltage of about 3.7 V. Therefore, in order to obtain a high output, it is necessary to take out a large electric current from a single battery whose size is increased. Consequently, there is a problem that a size of an entire apparatus is increased.
As a battery for solving these problems, a bi-polar battery has been proposed. The bi-polar battery is a battery having a structure in which a plurality of bi-polar electrodes are laminated in series with electrolyte layers being interposed therebetween and, in each of the bi-polar electrodes, a positive-pole active material layer is formed on one plate surface of a pyroelectric member and a negative-pole active material layer is formed on the other plate surface thereof. In this bi-polar battery, since the bi-polar electrodes are laminated in series within the single battery, a high voltage can be obtained in the single battery. Therefore, even when a high output is obtained, an output of a high voltage and constant current can be obtained. Furthermore, an electrical resistance of a battery connecting portion can be significantly reduced.
In the lithium-ion secondary battery, a structure using a liquid electrolyte is employed. However, in the bi-polar battery, since positive electrodes and negative electrodes are repeated in the single battery, the structure of the lithium-ion secondary battery using the liquid electrolyte cannot be applied to the bi-polar battery. That is, due to the structure of the bi-polar battery, it is necessary to take a structure in which electrodes are independently separated from one another, so as to prevent a short circuit (liquid junction) caused by ion conduction when electrolytes existing between electrode layers touch each other.
In the past, a bi-polar battery using a high polymer solid electrolyte including no liquid electrolyte has been proposed. In a case where this method is used, since the liquid electrolyte is not included in the battery, a possibility of a short circuit (liquid junction) caused by ion conduction between electrode layers is reduced. However, in general, an ion conductivity of the solid electrolyte is very low, for example, about 1/10 to about 1/100 of an ion conductivity of the liquid electrolyte. Due to this, a problem that the output density of the battery is reduced occurs, and thus, it has not been put to practice use.
In consideration of these circumstances, a bi-polar battery using a gel electrolyte obtained by semi-solidifying a liquid electrolyte is proposed. The gel electrolyte is a gel-like electrolyte in which an electrolyte solution is penetrated into a high polymer such as polyethylene oxide (PEO) or polyvinylidene fluoride (PVdF). The gel electrolyte is expected to obtain high ion conductivity and sufficient output density of a battery.
A problem remains to achieve an increase in a size of a bi-polar battery (high energy density). As a method for high energy density of a bi-polar battery, a method of increasing electrode areas of positive and negative electrodes and a method of connecting small-area bi-polar type single batteries in parallel to one another may be taken into consideration.
A lithium-ion secondary battery having a conventional electrode structure achieves high energy density by spirally winding positive and negative electrodes and a separator without a gap therebetween and filling the spirally winding into a battery outer case at a high density. However, since the positive electrode and the negative electrode are integrally formed in a structure of the bi-polar battery, counter electrodes are in contact with each other by the spiral winding. Therefore, there is a problem that a short circuit occurs if an insulating layer such as a separator or a polymer is not interposed between the bi-polar electrodes.